Olefin polymerization catalysts comprising a metallocene and an aluminum alkyl component were first proposed in about 1956. Australian patent 2436/88 proposed for use as a polymerization catalyst a bis-(cyclopentadienyl) titanium, zirconium, or vanadium salt as reacted with a variety of halogenated or unhalogenated aluminum alkyl compounds. Although some of these were capable of catalyzing the polymerization of ethylene, such catalytic complexes, especially those made by reaction with a trialkyl aluminum, had an insufficient level of catalytic activity to be employed commercially for production of polyethylene or copolymers of ethylene.
Later it was found that certain metallocenes such as bis-(cyclopentadienyl) titanium, or zirconium dialkyls in combination with aluminum alkyl/water cocatalyst formed catalyst systems for the polymerization of ethylene. Such catalysts are discussed in German Patent DE 2,608,863 published Sep. 9, 1977 which discloses a polymerization catalyst for ethylene consisting of bis-(cyclopentadienyl) titanium dialkyl, trialkylaluminum and water. German Patent DE 2,608,933 published Sep. 9, 1977 discloses an ethylene polymerization catalyst consisting of a cyclopentadienyl zirconium salt, a trialkylaluminum cocatalyst and water. European Patent Application No. 0035242 published Sep. 9, 1991 discloses a process for preparing ethylene and atactic propylene polymers in the presence of a cyclopentadienyl transition metal salt and an alumoxane. Such catalysts have sufficient activity to be commercially useful and enable the control of polyolefin molecular weight by means other than hydrogen addition--such as by controlling the reaction temperature or by controlling the amount of cocatalyst alumoxane as such or as produced by the reaction of water with an aluminum alkyl.
To realize the benefits of such catalyst systems, one must use or produce the required alumoxane cocatalyst component. An alumoxane is produced by the reaction of an aluminum alkyl with water. The reaction of an aluminum alkyl with water is very rapid, highly exothermic and pyrophoric in nature. Alumoxanes may be prepared by adding an extremely finely divided water, such as in the form of a humid solvent, to a solution of aluminum alkyl in benzene or other aromatic hydrocarbons. The production of an alumoxane by such procedure requires use of explosion-proof equipment and very close control of the reaction conditions in order to reduce potential fire and explosion hazards. For this reason, it has been preferred to produce alumoxane by reacting an aluminum alkyl with a hydrated salt, such as hydrated copper sulfate. In such a procedure a slurry of finely divided copper sulfate pentahydrate and toluene is formed and mantled under an inert gas. Aluminum alkyl is then slowly added to the slurry with stirring and the reaction mixture is maintained at room temperature for 24 to 48 hours during which a slow hydrolysis occurs by which alumoxane is produced. Although the production of alumoxane by a hydrated salt method significantly reduces the explosion and fire hazard inherent in the wet solvent production method, production of an alumoxane by reaction with a hydrated salt must be carried out as a process separate from that of producing the metallocene-alumoxane catalyst itself, is slow, and produces hazardous wastes that create disposal problems. Further, before the alumoxane can be used for the production of the an active catalyst complex the hydrated salt reagent must be separated from the alumoxane to prevent it from becoming entrained in the catalyst complex and thus contaminating any polymer produced therewith.
For many applications, such as gas phase polymerizations, it is desirable to have a supported catalyst. Supported metallocene-alumoxane catalysts have heretofore been produced by first adding the alumoxane to the support and then allowing the support to react with the metallocene. In copending applications U.S. Pat. Nos. 4,925,821 and 5,008,228 fully incorporated herein by reference disclose methods which are safer and more convenient for producing supported alumoxane metallocene catalyst. The methods disclosed in these U.S. patents provide catalysts which can be economically employed in a polymerization process. U.S. Pat. No. 4,925,821 describes a method of adding an undehydrated silica or U.S. Pat. No. 5,008,228 describes a method of adding a wet silica to a trialkylaluminum in a hydrocarbon solution. In each of these methods alumoxane is formed directly on the silica support in a safe and convenient manner. Thereafter, the catalyst is formed by depositing a metallocene on the alumoxane-containing support to yield a supported metallocene-alumoxane catalyst. The supported catalysts formed by such methods are highly active at conventionally utilized transition metal loading levels.
It is desirable to devise an economical procedure whereby a highly active supported metallocene-alumoxane catalyst could be safely produced for use as a gas phase or a slurry phase or a liquid phase polymerization catalyst. To be most economical, the procedure should not require the separate production of alumoxane.
Not only must the procedure be economical, but it is desirable to reduce the transition metal and/or aluminum content of the supported catalyst to as low a level as possible consistent with the level of activity needed for commercial viability. Reducing the transition metal and/or aluminum requirements decreases the raw material costs. In addition, the reduction in the level of the transition metal and/or aluminum serves to reduce the level of those catalytic metal constituents remaining in the polymer product as residue or ash. Where the transition metal and aluminum residue in the polymer exceeds about 1000 ppm, it is generally necessary to treat the polymer in a subsequent and expensive deashing step. In some applications, such as certain medical uses and in use in certain radiation environments, the presence of even very small quantities of transition metal and/or aluminum, which may leach out, could present a health problem. Therefore, it is desirable, in those applications, to have sufficiently low levels of the transition metal and/or aluminum, with or without deashing, to allow the polymers to be used in such applications.
Accordingly, there has been a continuing need in the art to discover catalysts of higher activity, specifically, in the context of supported catalysts. It has been a continuing desire to discover a catalyst which has a commercially useful rate of activity at the lowest loadings of active metals.